Ancient supervolcanoes revealed on Mars
Nature

A series of Martian craters assumed to have been formed by meteorites may actually be extinct volcanoes so massive that, when they were active billions of years ago, they could have buried Mars in ash.
The craters pepper the surface of Arabia Terra, a geologically ancient region of northern Mars. They appear as several huge circular pits that resemble Earth’s calderas, in which magma beneath a volcano drains after a volcanic eruption, causing the ground above the magma chamber to collapse. The best example on Mars is a feature called Eden patera, a depression about 85 kilometres long, 55 kilometres wide and 1.8 kilometres deep, says Joseph Michalski, a researcher jointly at the Planetary Science Institute in Tucson, Arizona, and at the Natural History Museum in London.